Freeform fabrication is fast becoming a popular process for manufacturing three-dimensional objects including finished products, prototype parts or models, and working tools. For example, freeform fabrication is used to make products such as structural ceramics and ceramic shell molds. Several methods of freeform fabrication involve a process of sequentially forming layers of the desired end product.
When freeform fabrication involves a process of sequentially forming layers, a number of planar layers are combined together to form a planar or non-planar, three-dimensional object. The object is formed layer-by-layer, with a segment of each layer representing a cross section of the final desired product. Adjacently formed layers are adhered to one another in predetermined patterns to build up the desired product.
In one freeform fabrication process, a powdery material is used to form each individual layer of the desired product. As shown in FIG. 1, a freeform fabrication unit (100) includes a supply of powdered build material. A measured quantity of the powder is dispensed from a supply chamber. A roller on a moving stage (103) distributes and compresses the powder at the top of a fabrication chamber (102). Then, a multi-channel jetting head, which may be based on inkjet printing technology, deposits adhesive or binder onto the powder in the fabrication chamber (102) in a two dimensional pattern. The jetting head may also be disposed on the moving stage (103). This two dimensional pattern is a cross section of the desired product. This jetting head may also eject ink or toner to provide a desired color or color pattern for that particular cross section of the desired product.
The powder becomes bonded in the areas where the adhesive is deposited, thereby forming a layer of the desired product. The process is repeated with a new layer of powder being applied over the top of the previous layer. The next cross section of the desired product is then formed into the new powder layer. The adhesive also serves to bind the adjacent layers of the desired product together.
This process continues until the entire object is formed within the powder bed in the fabrication chamber (102). The extra powder that is not bonded by the adhesive is then brushed away, leaving the base or “green” object. A user interface (104) allows a user to initiate and control the fabrication process.
Such a process offers the advantages of speedy fabrication and low materials cost. It is considered one of the fastest freeform fabrication methods, and can produce products in a variety of colors.
However, there are several disadvantages in conventional freeform fabrication processes including the fragility of the resulting product. Poor mechanical properties in the final product are a result of a low compression modulus which is due, in part, to poor powder adhesion. Poor mechanical properties are also manifested by fragility in extension, or low fracture strength. In both the intralayer and interlayer levels, the powder particles are only loosely glued together. More particularly, powders that are presently being used in the market are based on filler inorganic particles such as gypsum and/or plaster of Paris, etc., together with water swellable polymers such as starches, poly (vinyl alcohol), etc. and mixtures of these water swellable polymers.
When these types of systems are used, the powder surface is printed with an aqueous binder, and the polymer particles swell due to absorption of the aqueous binder. Adhesion is the result of the swelling of the polymer particles. Interaction of these powders with an aqueous binder results in poor mechanical strength as well as high porosity of the green object.
Also, parts made by powder-based freeform fabrication as well as jetted, direct build-up type freeform fabrication suffer from poor strength. The latter is due to the fact that only lower molecular weight polymers can be jetted since high molecular weight polymers have viscosities that are too high.
Further, the swelling process for the binding polymers of the above method tends to take place very slowly. The interaction between water and plaster of Paris also occurs very slowly. For these reasons, the conventional process requires more than an hour for the reacted materials to set and for the fabricated product to be removed from the powder bed.
Another problem that is directly associated with conventional powder-based freeform fabrication is the high density of the final product. The starting materials in the powder have such a high density that the prototype produced by the conventional process typically has a density that is greater than 1 g/cm3. The high density of the prototype is a serious nuisance, particularly when 1:1 scale models of large objects are being produced.
Further, the poor mechanical properties in the resulting product are related to the fact that the green object, which is fabricated by producing layers in a powder bed, must be subjected to labor intensive post-processing. This post-processing often involves soaking the surface of the printed object with reinforcing agents such as cyanoacrylate glue, etc. Gypsum based powders and water swellable polymers currently available require long swelling times, which can be thirty minutes or more. Another disadvantage of this and similar processes is that the resulting products can have a poor resolution, represented by a grainy texture of the product.
As mentioned above, the currently available processes for freeform fabrication use loosely bound polymer and inorganic particles to produce a product that has poor mechanical properties and a grainy texture. While post-processing drying of the resulting article improves the mechanical properties slightly, the improvements are minimal and the drying process is very slow. Other post-processing measures include reinforcing with polymerizable glues such as cyanoacrylate, or surface finishing, but these measures are costly and labor intensive.